What is the difference between "corporal punishment" and "spanking"?

I have seen questions about both of these. The ones that refer to schools using corporal punishment or parents everyone seems to be against it. Althought it seems the questions I have seen about spanking children most people seem to say it is “ok”.

My personal opinion is it is never ever acceptable for anyone to hit a child no matter how hard or for whatever reason, but why when it is worded “corporal punishment” it is wrong, but when it is “spanking” it is ok.

Don’t they both mean “hitting” ?